snippets: basic build system boilerplate

Basic things needed to integrate the new 'snippets' feature into the
build system. The main CMake variable which controls snippets is
SNIPPET. It is a whitespace-or-semicolon-separated list of snippet
names.

- Add minimal new cmake module for processing snippets. This just has
  basic infrastructure for processing a SNIPPET variable into
  SNIPPET_AS_LIST, and warning the user if they try to change it too
  late.

- Integrate the new module into the build system, via
  zephyr_default.cmake

This is anologous to the shields and boards modules' boilerplate and
input variables.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>

cmake/modules/snippets.cmake

@@ -0,0 +1,35 @@
+# SPDX-License-Identifier: Apache-2.0
+#
+# Copyright (c) 2021, Nordic Semiconductor ASA
+
+# Snippets support
+#
+# Outcome:
+# The following variables will be defined when this module completes:
+#
+# - SNIPPET_AS_LIST: CMake list of snippet names, created from the
+#   SNIPPET variable
+
+include_guard(GLOBAL)
+
+include(extensions)
+
+# Warn the user if SNIPPET changes later. Such changes are ignored.
+zephyr_check_cache(SNIPPET WATCH)
+
+# Putting the body into a function prevents us from polluting the
+# parent scope. We'll set our outcome variables in the parent scope of
+# the function to ensure the outcome of the module.
+function(zephyr_process_snippets)
+  if (SNIPPET)
+    message(STATUS "Snippet(s): ${SNIPPET}")
+  else()
+    set(SNIPPET_AS_LIST "" PARENT_SCOPE)
+    return()
+  endif()
+
+  string(REPLACE " " ";" SNIPPET_AS_LIST "${SNIPPET}")
+  set(SNIPPET_AS_LIST "${SNIPPET_AS_LIST}" PARENT_SCOPE)
+endfunction()
+
+zephyr_process_snippets()